1. Field of the Invention
The present invention relate to an exhaust gas purification apparatus for an internal combustion engine with a use of a catalyst.
2. Description of the Prior Art
In a lean burn engine for purification of exhaust gas, a NOx catalysis which is called NOx occlusion type or selective reduction type is used. A NOx occlusion catalyst stores NOx contained in the exhaust gas when air fuel ratio is lean and exhausts the stored nitrogen oxide NOx when air fuel ratio is rich, and thus hydrocarbon HC and carbon monoxide CO contained in the exhaust gas are purified by the reducing agent. This is to say that in a lean combustion engine, by arranging the NOx catalyst at some intermediate position of the exhaust pipe and performing repeated engine operation with lean air fuel ratio and the one with rich air fuel ratio, the NOx can be purified. For example, the exhaust gas purification apparatus for an internal combustion engine as disclosed by Japanese Laid Open Patent Application No. 2600492, in order to release nitrogen oxide NOx from the NOx catalyst during the operation of the engine operation with rich air fuel ratio with fully loaded, during the engine operation with stoichiometric air fuel ratio under being accelerated or when the occlusion capacity of the NOx catalyst is saturated, the internal combustion engine is driven in periodic engine operation with rich air fuel ratio and the one with stoichiometric air fuel ratio.
Also it is known that in a lean combustion engine, controlling an air fuel ratio to a predetermined air fuel ratio is performed depending on an operation state, such that during a partially loaded operation in order to improve the fuel consumption performance and the exhaust gas purification, the engine operation with lean air fuel ratio and the one with stoichiometric air fuel ratio are performed, and during a fully loaded operation, in order to assure a sufficient output, the engine operation with rich air fuel ratio is performed.
In an air fuel ratio control of a conventional lean combustion engine, because of direct shift to the engine operation with stoichiometric air fuel ratio from the one with lean air fuel ratio, it is possible that an incompletely purified NOx is released momentarily to the atmosphere as shown by FIG. 2E when the engine operation state is shifted to the engine operation with stoichiometric air fuel ratio from the one with lean air fuel ratio; this is supposed to be attributed to the factors {circle around (1)}xcx9c{circle around (3)} as follows: {circle around (1)} the NOx catalyst releases the occluded gas in the form of Ba (NO3)2xe2x86x92BaO+2NO+302/2. {circle around (2)} In the engine operation with stoichiometric air fuel ratio the reducing agent such as hydrogen carbon HC and carbon monoxide CO is in shortage. {circle around (3)} When the air fuel ratio stays near the engine operation with stoichiometric air fuel ratio, formation of NOx increases more than it dose under the engine operation with lean air fuel ratio.
The present invention is made in order to solve foregoing problems and provides an exhaust gas purification apparatus for an internal combustion engine with a capability of suppressing the release of the nitrogen oxide to the atmosphere when the engine operation state is shifted to the engine operation with stoichiometric air fuel ratio from the one with lean air fuel ratio.
According to the invention, an exhaust gas purification apparatus for an internal combustion engine comprising a NOx catalyst arranged in an exhaust gas passage of the internal combustion engine and an air fuel ratio control means in which one of operations among the engine operation with lean air fuel ratio, the one with stoichiometric air fuel ratio and the one with rich air fuel ratio is selected depending on an operation state, the air fuel ratio control means comprises a means for forced engine operation with rich air fuel ratio, for shifting the engine operation to the one with stoichiometric air fuel ratio after performing the engine operation with rich air fuel ratio when the operation state is shifted from the engine operation with lean air fuel ratio to the one with stoichiometric air fuel ratio.
According to the invention, in an exhaust gas purification apparatus for an internal combustion engine, a period during which the forced engine operation with rich air fuel ratio can be performed is dependent on a degree of richness.
According to an embodiment of the invention, in an exhaust gas purification apparatus for an internal combustion engine, the air fuel ratio under the forced engine operation rich air fuel ratio can be set to be within a range from 13.2 through 14.2.
According to an embodiment of the invention, in an exhaust gas purification apparatus for an internal combustion engine comprising a NOx catalyst arranged in an exhaust gas passage of the internal combustion engine and an air fuel ratio control means in which one of operations among the engine operation with lean air fuel ratio, the one with stoichiometric air fuel ratio and the one with rich air fuel ratio is selected depending on an engine operation state, the air fuel ratio control means comprises combustion fuel injection means for performing an additional fuel injection during an expansion stroke or an exhaust stroke when the operation state is shifted from engine operation with the lean air fuel ratio to the one with stoichiometric air fuel ratio.
According to an embodiment of the invention, in an exhaust gas purification apparatus for an internal combustion engine, a quantity of fuel to be additionally injected is set to be less than that of a reducing agent corresponding to the quantity of NOx occluded by said NOx occlusion catalyst.
According to an embodiment of the invention, in an exhaust gas purification apparatus for an internal combustion engine comprising a NOx catalyst arranged in an exhaust gas passage of the internal combustion engine and an air fuel ratio control means in which one of operation among the engine operation with lean air fuel ratio, the one with stoichiometric air fuel ratio and the one with rich air fuel ratio is selected depending on the engine operation state, the air fuel ratio control means comprises a means for forced engine operation with rich air fuel ratio for performing, when the engine operation state is shifted from the engine operation with lean air fuel ratio to the one with stoichiometric air fuel ratio, at first shifting the engine operation state to said engine operation with stoichiometric air fuel ratio after performing the one with rich air fuel ratio and performing thereafter an additional fuel injection during an expansion stroke or an exhaust strokes.